A lovely idea - it would make a great early Christmas gift, alongside a nice pen. It is a really good marketing opportunity.
I always use Diamine inks, but my one gripe with them is the difficulty in actually finding a shop that sells it. I live in London, the capital of the UK, where Diamine come from, one of the biggest cities in Europe, one of the great cities in the world.
To my knowledge (I'm happy to be corrected - please, please let me be wrong!) there is only one shop in the whole of London where you can buy Diamine ink - Greene and Stones in chelsea. And that shop is pretty out of the way.
They only stock a limited range of ten colours (sadly not the colour I most use) and although they will order different colours in for you, they will only do so when they next put an order into Diamine, which I was told can be as little as once every few months.
Yeah, you can buy it online, but ink in glass bottles in heavy and the postage can be almost as much as a bottle of ink. I'd rather just go to a shop and buy it for the cost of the ink (not the cost of the ink plus the same again for postage, making it twice as much as it should be), and have it in my hand - not have to order it, wait for a delivery, then find time to run to the post office to collect it because I'm not at home when it is delivered...
whinge whinge, gripe...
Sorry about the rant, but Diamine is one of the UK's success stories for fountain pens. Their marketing department seems non-existent though. The only ink you can get in most of the UK is Parker, Parker or Parker. Pen shops or larger stationery or Art shops might (might!) also have Cross, Sheaffer and one or two other inks from pen manufacturers - but never Diamine.
London no longer has many pen shops, but there are a few still, and the big department stores have pen departments. Surely Diamine can do something to get at least a basic range of colours into them?